'\" et
.TH CFGETISPEED "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
cfgetispeed
\(em get input baud rate
.SH SYNOPSIS
.LP
.nf
#include <termios.h>
.P
speed_t cfgetispeed(const struct termios *\fItermios_p\fP);
.fi
.SH DESCRIPTION
The
\fIcfgetispeed\fR()
function shall extract the input baud rate from the
.BR termios
structure to which the
.IR termios_p
argument points.
.P
This function shall return exactly the value in the
.BR termios
data structure, without interpretation.
.SH "RETURN VALUE"
Upon successful completion,
\fIcfgetispeed\fR()
shall return a value of type
.BR speed_t
representing the input baud rate.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
The term ``baud'' is used historically here, but is not technically
correct. This is properly ``bits per second'', which may not be the
same as baud. However, the term is used because of the historical
usage and understanding.
.P
The
\fIcfgetospeed\fR(),
\fIcfgetispeed\fR(),
\fIcfsetospeed\fR(),
and
\fIcfsetispeed\fR()
functions do not take arguments as numbers, but rather as symbolic
names. There are two reasons for this:
.IP " 1." 4
Historically, numbers were not used because of the way the rate was
stored in the data structure. This is retained even though a
function is now used.
.IP " 2." 4
More importantly, only a limited set of possible rates is at all
portable, and this constrains the application to that set.
.P
There is nothing to prevent an implementation accepting as an extension
a number (such as 126), and since the encoding of the Bxxx symbols is
not specified, this can be done to avoid introducing ambiguity.
.P
Setting the input baud rate to zero was a mechanism to allow for split
baud rates. Clarifications in this volume of POSIX.1\(hy2017 have made it possible to determine
whether split rates are supported and to support them without having to
treat zero as a special case. Since this functionality is also
confusing, it has been declared obsolescent.
The 0 argument referred to is the literal constant 0, not the symbolic
constant B0. This volume of POSIX.1\(hy2017 does not preclude B0 from being defined as the value
0; in fact, implementations would likely benefit from the two being
equivalent. This volume of POSIX.1\(hy2017 does not fully specify whether the previous
\fIcfsetispeed\fR()
value is retained after a
\fItcgetattr\fR()
as the actual value or as zero. Therefore, conforming applications should
always set both the input speed and output speed when setting either.
.P
In historical implementations, the baud rate information is
traditionally kept in
.BR c_cflag .
Applications should be written to presume that this might be the case
(and thus not blindly copy
.BR c_cflag ),
but not to rely on it in case it is in some other field of the
structure. Setting the
.BR c_cflag
field absolutely after setting a baud rate is a non-portable action
because of this. In general, the unused parts of the flag fields might
be used by the implementation and should not be blindly copied from the
descriptions of one terminal device to another.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIcfgetospeed\fR\^(\|)",
.IR "\fIcfsetispeed\fR\^(\|)",
.IR "\fIcfsetospeed\fR\^(\|)",
.IR "\fItcgetattr\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 11" ", " "General Terminal Interface",
.IR "\fB<termios.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
